This invention relates to novel pseudo-boehmites having excellent dispersibility and to processes for their production.
More particularly, the present invention is concerned with a pseudo-boehmite in the form of a thin film which is bulky and further characterized as having large pore volumes, large surface areas, and excellent dispersibility. It is concerned also with processes for the production of such products.
Methods and improvements in methods of producing alumina having a pseudo-boehmite structure from an alkali aluminate are known. In the best known method, an aqueous solution of alkali aluminate is neutralized with mineral acid to effect a continuous precipitation of a pseudo-boehmite. Various modifications have been proposed, particularly with respect to the kind and concentration of mineral acid, the concentration of alkali aluminate, and the reaction steps.
A method for preparing alumina monohydrate by reaction between basic aluminum acetate and an acid has been described. (See Japanese Patent Publication No. 14701/1967.)
In the first mentioned method, the reaction rate is increased due to the use of mineral acid with the result that the resulting boehmite has relatively coarse crystals, is poorly dispersible, and has a pore volume below 1.5 cc/g, generally 0.2 to 1.0 cc/g. When these products are dispersed in water, the large particles precipitate rapidly, and when employed as a filler for polymers and the like, the resulting products are not homogeneous.
The hydrate prepared by the second method has fine particles which have excellent dispersibility. However, the process is quite expensive to operate.
Heat dehydration of an amorphous aluminum hydroxide gel or aluminum hydroxide trihydrate also gives boehmite. In this case, however, the form of resulting product is a reflection of the form of the starting material. Normally, the product is nothing but a featureless crystal.
The reaction between alkali aluminate and halohydrin is referred to in Japanese Patent Publication No. 30180/1968, wherein is shown that the resulting products are spherical particles with fine, needle-like, radially grown crystals.